Hydraulic tensioners are commonly used to dampen excessive vibrations in the camshaft drive chain of an engine, more commonly termed the timing chain. Because the timing chain is subjected to torque reversals during normal operation, it is known for the tensioning device to incorporate a ratchet mechanism that allows a plunger tensioning the chain to move only in the direction to increase the tension in the chain. Ratchet tensioners of this type are disclosed in U.S. Pat. Nos. 6,547,684 and 6,612,951.
During installation of such a ratchet tensioner, the plunger needs to be fully retracted for later release after the tensioner has been mounted on the engine. If the ratchet mechanism were to be allowed to operate normally during installation, it would extend the plunger to the maximum of its stroke and prevent the tensioner from being installed.
Conventionally, the timing chain is housed in a casing with a removable cover which allows access to the chain tensioner while it is in situ in the engine. It is therefore possible to access the part of the tensioner that is within the casing to release the ratchet mechanism after it has been installed and both of the prior art references mentioned above incorporate ratchet release mechanisms that are suitable for use in such conventional engines.
More recently, however, it has been proposed to form engine castings in such a manner that the chamber within which the timing chain is encased forms an integral part of the engine block and the engine cylinder head and there is no cover that can be removed to expose the chain or the chain tensioner.
The present invention seeks to provide a ratchet tensioner that may be used in such engine.
According to the present invention, there is provided a tensioner for a chain enclosed within a casing, the tensioner comprising a body for mounting on the exterior of the casing, a plunger slidably mounted in the body and urged in use into the interior of the casing to deflect a run of the chain laterally in order to increase tension in the chain, wherein the plunger is formed with ratchet teeth which interact with a pawl that is pivotably mounted on the body to permit movement of the plunger only in the direction to increase chain tension, wherein a device is provided to act on the pawl so as to inhibit pivoting movement of the pawl and thereby prevent movement of the plunger during installation of the tensioner, said device operable from outside the casing in order to free the pawl after the tensioner has been mounted on the casing.
In the preferred embodiment of the invention, the device for inhibiting pivoting movement of the pawl comprises a pin that is retractable from outside the casing.
The plunger may be urged in the direction of the chain by a spring or a hydraulic pressure but more preferably by both.
The pin is advantageously arranged to act on the pawl by way of a spring that is fully compressed by the pin during installation, the spring serving to bias the pawl resiliently into engagement with the ratchet teeth on the plunger when the pin is retracted.
The pin may be retracted by a cross bolt mounted in the body transversely to the axis of the pin, the bolt having a necked or waisted shank comprising a large diameter portion connected by a conical region to a reduced diameter portion.
When the end of the pin contacts the cylindrical surface of the large diameter portion of the shank, it is pushed into the position in which it locks movement of the pawl. However, as the bolt is driven home into the body, the end of the pin becomes aligned first with the conical region then with the reduced diameter portion of the shank and is thus allowed to retract away from the pawl. The conical region makes the transition gradual and allows the pawl to be re-locked if necessary.
The use of a cross bolt offers the advantages that it can readily be screwed tight using powered assembly tools and that it provides visual confirmation that the ratchet mechanism has been released.